A kinetic study of the liquid phase naphthalene hydrogenation over a commercial and sulphided NiMo/gamma-Al2O3 was performed using experimental data obtained in a Robinson-Mahoney reactor at 523-583 K and 2.0-4.0 MPa. The effect of H2S on the hydrogenation rate was investigated varying its partial pressure from 0.09 to 0.89 MPa. Under these conditions, an exponential decrease in the hydrogenation rate was observed as the H2S partial pressure increased. The naphthalene conversion ranged from 6 to 89% with tetralin as the main hydrogenation product with selectivities of at least 89%. Reaction networks were formulated based on stepwise hydrogenation mechanisms assuming that both hydrogen and hydrogen sulphide could be dissociated either homolytically or heterolytically on both coordinatively unsaturated metal ion sites (CUS) and sulphur anions (SA) of the sulphided catalyst. Langmuir-Hinshel wood rate equations were derived and discriminated through model regression to experimental data. Two models best describe the experimental data according to a statistical analysis and the physical meaning of the parameter estimates. These models correspond to the third hydrogen addition chemisorbed on CUS and supplied from either homolytic or heterolytic hydrogen dissociation, as rate-determining step. The calculated catalyst surface species concentrations indicate that hydrogen and sulphydril groups are the most abundant species under the investigated operating conditions while the hydrocarbons surface concentration is almost negligible. (c) 2007 Elsevier B.V. All rights reserved.